A 'conditional conversion' option was recently added to NConvert, as described in this topic:
Option to 'Convert only if specified file parameter condition is satisfied'
A similar option in XnConvert would be a useful enhancement, and might possibly consist of a panel on the Output tab labelled 'Conditional conversion' containing a check box to enable the option, and suitable drop down menu(s) and/or radio button(s)...

Recent example request:
Keep original image if file size is smaller